I posted an example of damage in another thread, and then got to thinking about the Severity of critical hits in space combat,
pg 158
The Severity of the critical hit is equal to the damage
the spacecraft has taken from the attack, divided by
ten (rounding up)

Let's look at 2 attacks from the Harrier ship against both types of Far Trader. The first is the particle barbette; 4D at very long range (plus a radiation hit).
Assume a 6 Effect to get a critical hit, then a lousy damage rol to give us a damage of 14.

This gives a Severity roll of 1.4 or 1.2 against the ships (0 or 2 Armour)
Rounding up gives us a Severity 2 critical hit? Anything over 1.0 is a 2 Severity hit? Is that right?

Second scenario is a 3 missile salvo. Effect of 3 on the attack roll. Average damage is 14, times 3 is 42. Does the 4.2 allow for a level 5 severity critical hit on the armour 0 ship?

PsiTraveller wrote: ↑
This gives a Severity roll of 1.4 or 1.2 against the ships (0 or 2 Armour)
Rounding up gives us a Severity 2 critical hit? Anything over 1.0 is a 2 Severity hit? Is that right?

Yes, an effect of 6 and a single point of damage is Severity 1; more damage means higher Severity.

PsiTraveller wrote: ↑
Second scenario is a 3 missile salvo. Effect of 3 on the attack roll. Average damage is 14, times 3 is 42. Does the 4.2 allow for a level 5 severity critical hit on the armour 0 ship?

With that in mind it would make sense to spend an extra 8 Million Credits for Fire Control 5. This would give a +5 DM to the particle barbette, overcoming the -4 DM for shooting from very long range.
Assuming a Gunner 2 and Dex +1 DM that gives a +4 DM when attacking from long range. This will only give an Effect 6+ Critical Hit on a roll of 10+, but otherwise there is no chance to get a critical hit from Very Long range.

The +4 to the attack roll means that average damage will be 17 instead of 14. Nice.

For missile attacks the TL 15 will give a bonus vs lower tech ships. +3 for most TL 12 ships. Add in any extra missiles if you added more missiles to the salvo and you can do some nasty damage.

Good point. I'm just trying to overcome the -4 DM from very long range. The +2 from Fire control 2 is helpful for the barbette, and not needed for the missiles (unless you wanted the computer to shoot for you.

I remember the combat from the Drinax game was always very vicious in the first 2 or 3 rounds and then the fights were generally over. The radiation hits on a smaller crew make combat difficult after half the crew is down.

A solid hit on light ships can start a cascade failure on the hull as the cumulative damage kicks in.

About that cascade, in our group we've added a house rule which separates damage from critical hit effects - it doesn't count against sustained damage (but can still kill the ship). This way smaller ships are protected from the death-cycle.

I remember our first scout. Someone hit us for 4 damage (with beam laser), then we rolled 7, took severity 1 on hull which triggered another 1D (rolled 5), which triggered another critical, where we rolled 7 again (severity 2 hull), which triggered.... At the end we rolled something else, and the ship was left with 1 hull point.

That is kinda odd as to get a critical hit from an attack the attacker needs a 6+ effect, further to get a sustained damage critical you need to score 10+ or 1% of hull value. ( Which ever is greater.)

That is kinda odd as to get a critical hit from an attack the attacker needs a 6+ effect, further to get a sustained damage critical you need to score 10+ or 1% of hull value. ( Which ever is greater.)

Not quite.

You can achieve critical hits in three different ways:
Sustained: Every time the target passes an even 10% of total damage, regardless how much damage in the attack.
Small ship: Effect 6+ & damage at least 1. Severity: damage / 10 (round UP)
Large ship: Effect 6+ & damage at least 1% of target total (min 10). Severity: damage / 1% (round DOWN)

The further note is the damage needs to exceed the armour value in order for a crit to occur in any of the cases.

If an attack roll against a spacecraft has an Effect of
6 or higher and it causes damage (rather than just
bouncing off armour), a critical hit has been scored

I also have a feeling the 1% or 10 points minimum is a correction to the main book formulation so as to have a single consistent scale as the 10% round up allows too great a jump in severity. Not to mention having two different systems for determining sustained damage severity is rather unnecessary.

Core, p158 wrote:Critical Hits
If an attack roll against a spacecraft has an Effect of 6 or higher and it causes damage ...

A lucky hit can cause an immediate crit with a variable Severity depending on how much damage was inflicted.

Core, p158 wrote:Sustained Damage
The systems on a ship are not only vunerable to lucky hits. Sustained, continued punishment will cause failures as well.

In addition, for every 10% damage the ship takes, it takes a Severity 1 crit, regardless of attack rolls.

The first case, the lucky hit, is modified in HG for "large" ships (whatever "large means):

HG, p23 wrote:Critical Hits on Large Ships
...
Large ships can also endure a great deal more damage before the effects of any critical hits become noticeable. The Severity of a critical hit is based on 1% increments of the ship’s hull value (minimum 10 points of damage). For example, a ship with 10,000 Hull points that receives a critical hit that causes 224 points of damage, will sustain a Severity 2 critical hit.

So, you can achieve critical hits in three different ways:
Sustained: Every time the target passes an even 10% of total damage, regardless how much damage in the attack or the attack roll.
Small ship: Effect 6+ & damage at least 1. Severity: damage / 10 (round UP)
Large ship: Effect 6+ & damage at least 1% of target total (min 10). Severity: damage / 1% (round DOWN)

Taking a digression from rules for a moment, note that a 50 kg missile at Traveller space combat velocities should be devastating if it hits anything. Even one minute at 1 G is ½ × 9.8m/s^2 × (60s)^2 = 17.64 km/s. That's likely to create a missile-diameter cylinder of plasma from the first point of contact on the ship, all the way through, and a great distance out the far side of the ship until the ionized vapor of missile and ship disperses into space.

You really don't want a missile-diameter cylinder of plasma to suddenly appear in a path through your ship. It tends to cause heat damage to the components of the ship in which the plasma cylinder appears, even if the portion of the ship that is turned to plasma manages not to include parts of the ship that are necessary for structural integrity or normal operations.

By that reasoning, if the rules provide for anything other than a critical hit from a missile hit, they're reducing the damage for dramatic value.

Note: I'm not actually sure how much damage a 17 km/s impact does. I'm extrapolation from articles about experimental Gauss weapons that could shoot a 1 cm cube of aluminum (2.7 grams) at 5 km/s, through at least 20 cm of tank armor grade steel, which means vaporizing 150 to 160 grams of armor to make the hole. Traveller (other than the GURPS and possibly Fire, Fusion, and Steel editions) is vague about the thickness and density of ship armor and internal components, so I'm not sure how much of the missile and ship would carry through the hole in a ship.

I know better, which makes that a pretty silly mistake. The 600 m/s of 1 minute at 1 G is not going to be devastating, though it would hurt. But turns are 6, 10, or 20 minutes (depending on edition), acceleration is 6 G to 15 G or more. Even 6 minutes at 6 G is 21 km/s, which is pretty much "cylinder of plasma" territory, after just one turn.

The Stardust space probe intentionally exposed itself to high velocity impacts (6.5 km/s, low for interplanetary space); the largest object that struck it was about 1 mm across. They were captured in blocks of aerogel either 1 cm or 3 cm thick, and penetrated as much as 200 diameters deep. Since 1 mm × 200 is 20 cm, either that grain blew through or stopped well before 200 diameters.

Anyway, if cometary and interstellar dust can penetrate 200 diameters into extremely low density stuff like aerogel (a solid with density similar to air), how far can a much higher velocity missile penetrate starship armor? I don't know, but I think it's a question of momentum, supersonic friction, and turbulence. Without the necessary physics knowledge, my guess is that the plasma cylinder is a likely result.